This invention relates to a digital signal processing circuit for filtering of thinning and interpolating an image signal to be compressed and recorded as a digital signal.
Throughout the specification, the term thinning and interpolating are used to represent different types of vertical filtering.
It is well known that of an image signal to be input is compressed and recorded as a digital signal. Particularly, where an image signal is recorded as a digital signal to a videotape for general use, the information must be compressed to one tenth. In general, for compression of the image signal, which is composed of the luminance signal and the color-difference signals, the color-difference signals are compressed.
Generally, in the United States and Japan, the NTSC method is used for signal transmission; in Europe, the PAL method is the most commonly used signal transmission method. Since each method has a different transmission system for the luminance signal and the color-difference signals, each method has a different compression method for compressing the image signal (mainly the color-difference signals). The transmission systems For the luminance signal and the color-difference signals at one image in the NTSC method and the PAL method are respectively shown in FIGS. 4(a) and (b). In the respective methods, a signal at one image is generally composed of luminance signals Y and two kinds of color-difference signals Cr and Cb. The color-difference signals Cr, Cb are distributed alternatively in the horizontal direction of the image. The number of horizontal picture elements of the luminance signal Y is 858 in the NTSC method and 864 in the PAL method. The number of perpendicular lines is 525 in the NTSC method and 625 in the PAL method. The image signal is generally compressed in such a manner that the color-difference signals Cr and Cb are thinned with a signal rate of 1/2, the thinned color-difference signals Cr and Cb are serially translated according to DCT (discrete cosine transform) or the like together with the luminance signal Y, then a sign indicating the serially translated signal is translated so as to be a sign whose entropy is minimum. For thinning with the signal rate of 1/2, the color-difference signals Cr and Cb are thinned in the horizontal direction of the image as shown in FIG. 5(a) in the NTSC method, and the color-difference signals Cr and Cb are individually thinned in a perpendicular direction at every two lines as shown in FIG. 5(b) in the PAL method. The reason the color-difference signals Cr and Cb are thinned in the perpendicular direction of the image in the PAL method is because it is hard to cause degradation of the quality of the image because the perpendicular lines in the image in the PAL method are 100 lines more than that in the NTSC method.
When the color-difference signals are thinned, the picture element value before thinning is not used directly as a color-difference signal value (picture element value) Y after thinning, but the color-difference signal value Y after thinning is obtained by reflecting three successive picture element values before thinning to the picture element value Y after thinning. A formula (1) for calculating the picture element value Y is often used. EQU Y=X.sub.2 /4+z.sup.-1 X.sub.1 /2+z.sup.-2 X.sub.0 /4 (1)
X.sub.n : picture element value of the input signal PA1 z.sup.-1 : delay of z-field PA1 (NTSC method: one picture element) PA1 (PAL: one horizontal line) PA1 X.sub.n : picture element value of the input signal PA1 z.sup.-1 : delay of z-field PA1 (NTSC method: one picture element) PA1 (PAL method: one horizontal line)
In the case where the thinned image signal is recorded to a tape or the like and the recorded thinned signal is regenerated, it is necessary to interpolate between the recorded color-difference signals. The picture element value Y of an interpolated color-difference signal located between the recorded color signals (thinned signals) is generally calculated with formula (2) below, using successive thinned signals (X.sub.1 does not exist in formula (2)). EQU Y=X.sub.2 /2+z.sup.-2 X.sub.0 /2 (2)
In order to put formulas (1), (2) into practice, generally the input signal is delayed by two delay circuits D provided as shown in FIGS. 6(a) and (b).
With the constructions for signal thinning in FIG. 6(a) and for signal interpolation in FIG. 6(b), however, the circuit size is enlarged in the
method, while the circuit size is small in the NTSC method.
In detail, in the NTSC method, when three successive color-difference signals are sequentially input the picture element value after thinning is calculated using formula (1) because the color-difference signal is thinned in the horizontal direction of the image. Thus, only two delay circuits for one picture element (usually two D flip-flops) suffice. In the case of interpolation using the signal after thinning, when two thinned signals are input sequentially, the picture element value after interpolation is calculated using formula (2). Thus, only two delay circuits for one picture element suffice as well as for the thinning. On the other hand, in the PAL method in which the color-difference signal is thinned in the perpendicular direction of the image, the picture element value after thinning is calculated using formula (1), only when the color-difference signals of three successive lines are sequentially inputted. Also, for the interpolation, the picture element value after interpolation is calculated using formula (2), only when the color-difference signals of two lines after thinning are input sequentially. Therefore, two delay circuits for storing and delaying the color-difference signals by one line are required. In detail, as shown in FIG. 4(b), since the number of effective picture elements for one line is 720 (excluding 144 picture elements corresponding to an eclipsing region from all of 864 picture elements in one line) in the PAL method, two 360-stage FIFO memories (first in first out buffer memory) are usually required for the color-difference signal Cr and the color-difference signal Cb respectively. This causes the circuit to be enlarged. The circuit is enlarged not only in the PAL method, but also when an image signal is thinned in the perpendicular direction for recording and the thinned signal is interpolated at regeneration of the recorded image signal.